In completing an oil and/or gas well, the cased borehole is perforated at the payzone by shooting holes or perforations through the casing, the surrounding cement, and into the hydrocarbon formation to permit the hydrocarbons to flow into the cased borehole and up to the surface. It is often desirable to place a predetermined underbalance or a predetermined overbalance on the formation. In doing so, a tubing string with a vent assembly, and generally a perforating gun if the well is to be perforated, is lowered into the cased borehole. A hydrostatic head is placed inside the tubing string to establish the predetermined underbalance or overbalance on the formation. Such a method and apparatus have several applications depending upon the particular environment.
After a well has been produced for many years, geologists skilled in the art of interpreting downhole logs sometime discover that the producing formation should be reperforated to create additional perforations to increase production. Often in the prior art, a through-tubing perforating gun had originally been used instead of a casing gun such that the original perforations were not deep enough to adequately produce the formation. Also, sometimes metal shots such as bullets were used for perforating rather than shaped charges such that good perforations were not achieved. In such cases, the perforations may not have reached the sterile formation and it becomes desirable to reperforate the formation to increase production.
It may also be discovered that additional perforations should advantageously be formed in the casing at a location spaced above or below the old perforations. This can often significantly increase the production from the old formation. Sometimes, the formation was missed completely at the time of the first perforation where the perforations were too high or too low with respect to the payzone.
Moreover, it sometimes happens that other payzones closely adjacent to the older perforated zones have been overlooked, and in those instances, it is often desirable to place additional perforations into the suspected new payzone without unduly disturbing the old perforations.
The above proposed new perforations often must be formed into a highly unconsolidated formation. Accordingly, care must be taken during reentry and reperforation not to unduly distrub the old formation, or otherwise it is possible to injure the production area of the borehole to the extent that costly equipment must be brought onto location in order to repair the damage. It may be desirable that the new perforations be made in such a manner that the unconsolidated zone is not produced along with the production fluid from the old payzone.
At other times, it may be suspicioned that the prior perforations insufficiently communicated the payzone with the lower annular end of the borehole, and that employment of modern techniques might improve the production of the wellbore.
Sometimes, deposits have formed in the old perforations whereby the perforations become partially plugged. One objective might be to resurge the old perforations, without reperforating, in an attempt to increase production.
In some instances, it is desirable to have an overbalance on the producing formation, i.e. the hydrostatic head inside the tubing string is greater than the formation pressure. In that case, pressure is effected into the producing formation momentarily, thereby preventing damage to the producing payzone. Otherwise, the unconsolidated material from the payzone could flow into the cased borehole, into the tubing string, and uphole towards the surface which is undesirable in this instance.
It may also be desirable to have an overbalance such that upon perforating, hot acid in the bottom of the tubing string is forced into the formation.
In completing a new well where the payzone is a highly unconsolidated sand formation, it is sometimes desirable to have only a small pressure differential toward the tubing string. Thus, it is desirable to reduce the pressure differential to a predetermined low value to control the backsurge on the new payzone.
To achieve the desired underbalance or overbalance, various types of vent assemblies are employed to open and close the interior of the tubing string to fluid flow. One such vent assembly is shown in U.S. Pat. No. 4,299,287.
In controlling the differential pressure, it is necessary to establish a hydrostatic head in the tubing string whereby the tubing string is filled with fluid to a predetermined level. Normally, the tubing string would be filled with kerosene, diesel, nitrogen or water. The perforating gun is generally detonated by a bar and the fluids in the tubing string slow the descent of the bar as it drops through the tubing string to detonate the perforating gun. To open a bar actuated vent assembly, there must not only be sufficient impact on the vent assembly to move it into the open position, but there must be approximately 22 ft-lbs of impact for the bar to actuate the firing head. If the bar does not open the vent assembly and detonate the gun, it becomes necessary to fish the bar out of the well. Further, it is possible that the bar might get hung up on the vent assembly. If well fluids are used in the tubing string, mud and other materials may settle to the bottom so as to not only slow the descent of the bar but possibly stop the descent altogether if the debris becomes compacted.
Apparatus and method for overcoming the above-identified completion problems are the subject of this invention.